This invention relates to poly(B-alkenyl-borazine) ceramic precursors, to the novel B-alkenyl-borazines from which such ceramic precursors are made, to methods of synthesizing such ceramic precursors and B-alkenyl-borazines, and to methods of using such precursors to form boron nitride ceramics.
Pyrolysis of polymers to produce various carbon products, especially fibers, is widely practiced. One of the most important applications of this technology is in the preparation of carbon-carbon composites, where it is generally used to prepare both the fibers and the matrix. Carbon fibers made by pyrolysis were used in the first successful ceramic fiber composites and continue to be of interest in such applications. The lower costs, high strength and high Young's moduli of carbon fibers are strong attractions. However, these fibers are not very resistant to oxidation, and therefore their use in ceramic composites is limited. As a result, SiC-based fibers obtained by pyrolysis of polycarbosilanes have become the predominant material for use in ceramic fiber composite research and development studies. Because of these and other high temperature environmental capabilities, an extensive interest in SiC-based fibers has occurred, even though strengths are typically lower and Young's moduli are substantially lower than those of most carbon fibers.
It has been pointed out that a variety of different precursors are needed-if one is to produce a variety of ceramic compounds and/or mixtures for different applications. Rice, R. W., Am. Ceram. Soc. Bull., 1983, 62, 889-892. Such applications include bulk materials, matrices, fibers and coatings. Wynne, K. J., Rice, R. W., Ann. Rev. Mater. Sci., 1984, 14, 297-334 have pointed out that the polymer pyrolysis technique offers several advantages over other preparative techniques, and that the technique is especially suitable to syntheses of silicon carbide and silicon nitride. The same article suggests that preceramic pyrolysis chemistry should find applications in the formation of boron nitride (BN) materials; however, until recently, little progress has been made in attempts to produce BN solid state materials in this fashion.
Polymers which yield boron nitride, BN, belong-to one of the most important sets of precursors because BN and graphite have the same structure and bonding. Due to this similarity, BN has stiffness, strength and ablation characteristics that are comparable to those of graphite. On the other hand, BN, an excellent dielectric material, has a very low dielectric constant and a low dielectric-loss factor in contrast to the high values of these two properties of graphite. BN also has a greater resistance to oxidation than graphite. Thus, BN provides the potential of making an oxidation-resistant body with strengths, stiffnesses, and ablation resistances comparable to those found in graphite.
BN polymer precursors are also of interest in the preparation of BN fibers, coatings, or as a matrix in ceramic-ceramic fiber composites. As a matrix for composites, it could be used with a variety of fibers. For instance, if one desired a composite with good dielectric properties, the use of fibers and matrix with good dielectric properties would be desirable. While fibers different from the matrix might be considered for this application, it is clear that BN fibers with a BN matrix would be preferred because of comparable dielectric behaviors.
There have been attempts to utilize polymer pyrolyses as low temperature alternatives to the preparation of BN. Inert atmosphere pyrolyses of various borazine derivatives, for example, led to reasonable yields of ceramic materials containing boron and nitrogen, but the materials also contained substantial amounts of carbon. Bender, B. A., Rice, R. W., Spann, J. R., Ceram. Eng. Sci. Proc., 6, 1171-1183 (1985). Narula, C. K., Paine, R. T., and Schaeffer, R. Mat. Res. Soc. Symp. Proc., 73, 383-388 (1986) report that oligomerization reactions of substituted borazines with silylamine crosslinking groups have been found to provide useful gel materials which upon pyrolysis form boron nitrogen materials. Other borazines indicated as possibly being suitable for pre-ceramics production include B-trianilinoborazine, B-triamino-N-triphenylborazine, and B-triamino-N-trimethylborazine. Pyrolysis of such compounds failed, however, to give pure boron nitride; carbon was invariably retained. Paciorek, K. J. L., Harris, D. H., Kratzer, R. H., J. Polym. Sci., Polym. Chem. Ed., 1986, 24, 173-185. Polymers of tert. aminoborazine, anilinoborazine, phenylaminoborazine and aminoborazine were pyrolyzed in attempts to produce BN in Bender, B. A., Rice, R. W., Spann, J. R., Cer. Eng,. Sci. Pro., 1961, 55, 153-160. Pyrolysis of the tert. aminoborazine gave no yield whatsoever, indicating complete decomposition to volatile species. The other three precursors gave measurable yields, with phenylaminoborazine and aminoborazine giving quite practical yields. The physical appearance of all the resultant pyrolysis products was dark gray to black, indicating a substantial carbon content.
The synthesis of BN from ammonia pyrolysis of soluble polyborazine compounds has been disclosed by Paciorek, K. J. L., Harris, D. H., Schmidt-Krone, W., and Kratzer, R. H., Technical Report No. 4, 1987, Ultrasystems Defense and Space Inc., Irvine, Calif. In addition, it has recently been reported that ammonia pyrolysis of decaborane polymers linked by diamine molecules produced crystalline BN of high analytical purity. (Presentation of Rees, W. S., Jr. and Seyferth, D. at the 194th National Meeting of the American Chemical Society, September 1987, Paper INOR 446; J. Am. Ceram. Soc., 71 [4] C-194-C-196 (1988).)
There remains a clear need for a BN polymer precursor which will provide BN in high yields and high purity. Ceramic precursors which are processible are also greatly desired because they would allow the ceramic to be used in a variety of applications not presently commercially feasible. For example, if a soluble precursor to B.sub.4 C were available, thin films of the solubilized precursor could be cast and pyrolyzed to yield thin films of the ceramic material. Similarly, a variety of substances could be coated with the soluble precursor material by various dipping or spraying techniques to yield, after thermal annealing, a substrate coated with the desired BN or B.sub.4 C ceramic material. The soluble precursor might also be used to prepare spun fibers of ceramic material or in preparing a multitude of various ceramic/fiber composites.
The polymerization of certain olefinic boron-containing compounds was studied in Pellon, J., Deichert, W. G., Thomas, W. M., J. Polym. Sci., 55, 153-160 (1961). In particular, the polymerization of B-triallyl-N-triphenyl borazine (TAB) and B-trivinyl-N-triphenylborazine (TVB) was studied, and it was found that while TAB would homopolymerize, it was not very reactive, and TVB would not polymerize at all. It was postulated that the low or absent reactivity of these compounds was due to the steric effect of the flanking phenyl groups. The authors noted that-they would have preferred to use a monovinylborazine with hydrogen on the adjacent nitrogens, but that synthetic difficulties had prevented that approach.
To date, Applicants are unaware of any publications relating to a method of synthesizing mono- or di-alkenylborazines. The synthesis of perfluorovinylborazines such as B,B'-dimethyl-B"-perfluorovinyl-N, N', N"-trimethylborazine and B,B', B"-tris(perfluorovinyl)-N,N', N"-trimethylborazine is disclosed in Klancia, A. J., Faust, J. P., King, C. S., Inorg. Chem., 1967, 6, 840-841. Klancia, A. J. and Faust, J. P., in Inorg. Chem., 1968, 7, 1037-1038, disclose a method for synthesizing B-vinylpentamethylborazine. The preparation of B-trivinylborazine by bubbling ammonia gas through bis(dimethylamino)vinylborane is disclosed in Fritz, P., Niedenzu, K., and Dawson, J. W., Inorg. Chem., 1964, 3, 626-627.
A method for preparing mono-vinyl borazines by reacting borazine with acetylene in the presence of a transition metal catalyst such as Ir(CO)Cl [P(C.sub.6 H.sub.5).sub.3 ].sub.2 is disclosed in Sneddon, L. G., Pure & Appl. Chem., 59, Vol. 7, pp. 837-846 (1987). A similar reaction using RhH(CO)PPh.sub.3).sub.3 is disclosed in Lynch, A. T. and Sneddon, L. G., J. Am. Chem. Soc., 1987, 109, 5867.
It is an object of this invention to provide ceramic precursor polymer materials rich in boron and nitrogen which can be pyrolyzed under mild conditions to yield BN ceramics in high purity and high yields. It is a further object of this invention to provide such ceramic precursors which are processible. These and other objects will be made clear from the following summary and discussion of this invention.